We report on the free tropospheric spatio-temporal variability of water vapor investigated by the analysis of a five-year period of water vapor vertical soundings above Mt. Zugspitze (2962 m a.s.l., Germany). Our results are obtained from a combination of measurements of vertically integrated water vapor (IWV), recorded with a solar Fourier Transform InfraRed (FTIR) spectrometer and of water vapor profiles recorded with the nearby differential absorption lidar (DIAL). The special geometrical arrangement of one zenith-viewing and one sun-pointing instrument and the temporal resolution of both optical instruments allow for an investigation of the spatiotemporal variability of IWV on a spatial scale of less than one kilometer and on a time scale of less than one hour. We investigated the short-term variability of both IWV and water vapor profiles from statistical analyses. The latter was also examined by case studies with a clear assignment to certain atmospheric processes as local convection or long-range transport. This study is described in great detail in our recent publication [1]. 1. INSTRUMENTATION AND GEOGRAPHICAL ARRANGEMENT The Zugspitze (47:42 ◦ N, 10:98 ◦ E, 2962 m a.s.l.) is by far the highest mountain on the northern rim of the Alps. The free troposphere above this site is representative of Central Europe. The mountain is above the moist boundary layer for most of the year. Due to reduced absorption losses this site is ideal for sensitive spectroscopic measurements of water vapor throughout the free troposphere. While the FTIR instrument is located on the summit of Mt. Zugspitze the DIAL instrument is located at the Schneefernerhaus research station (UFS) on the steep southern slope of Mt. Zugspitze at an altitude of 2675 m a.s.l., 680 m southwest of the FTIR instrument (Fig. 1). The solar FTIR instrument uses direct radiation from the sun in the mid-infrared as light source (Table). The instrument provides water vapor columns integrated along its slanted viewline towards the sun with a precision better than 0.05 mm. It is based on a Bruker IFS125HR interferometer and is described in detail by [2].